Method and arrangement for transmitting and receiving rf signals through various radio interfaces of communication systems

ABSTRACT

A method and arrangement for transmitting and receiving RF signals, associated with different radio interfaces of communication systems, employ a direct conversion based transceiver which substantially comprises one receive signal branch and one transmit signal branch. Mixing frequencies of the different systems are generated by a single common by use of an output frequency divider in combination with the synthesizer, and by use of filtering corresponding to a system channel bandwith by means of a controllable low-pass filter operating at baseband frequency.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No. 09/856,746 filed May 24, 2001, which is a U.S. national stage of PCT/FI99/00974, filed on Nov. 25, 1999, which is based on and claims priority to Finish application no. 982559, filed on Nov. 28, 1998, all incorporated by reference herein.

The invention relates to a method and arrangement for transmitting and receiving RF signals associated with various radio interfaces of communication systems. The invention finds particular utility in transceivers of general-purpose mobile stations. Mobile communication systems are developing and expanding rapidly which has lead to a situation in which there are in many areas systems complying with several different standards. This has brought about a need for mobile stations that can be used in more than one system. Good examples are the digital systems called GSM (Global System for Mobile communications) and DCS (Digital Cellular System), which operate on different frequency bands but have otherwise similar radio interfaces. In addition, the modulation, multiplexing and coding schemes used may be different. The systems mentioned above use the time division multiple access (TDMA) method; other methods include the frequency division multiple access (FDMA) and code division multiple access (CDMA).

One possible way of making a mobile station capable of operating in multiple systems is to have in the mobile station completely separate signal paths for each system. This, however, would lead to an unreasonable increase in the mobile station size and manufacturing costs. Therefore, the goal Is to design a mobile station in which the differences relating to the radio interfaces of the various systems could be largely dealt with by means of programming, instead of having separate signal processing paths.

It is known e.g. from patent application document EP 653851 a transceiver arrangement using one local oscillator the frequency of which falls between the lower operating frequency band and the higher operating frequency band such that one and the same intermediate frequency (IF) can be used for both operating frequency bands. However, the disadvantage of such a solution is that the necessary IF stages make the Implementation rather complex, and the manufacturing costs of the device will be high because of the great number of components. Furthermore, the IF stages require filters in order to eliminate spurious responses and spurious emissions. In addition, channel filtering at the intermediate frequency sets great demands on the IF filters.

In a direct-conversion, or zero-IF, receiver the radio-frequency (RF) signal is directly converted into baseband without any Intermediate frequencies. Since no IF stages are needed, the receiver requires only a few components, therefore being an advantageous solution for general-purpose mobile stations which have multiple signal branches for different systems. To aid in understanding the problems relating to the direct conversion technique and prior art it Is next described in more detail a prior-art solution.

FIG. 1 shows a direct conversion based arrangement for realizing a dual frequency band transceiver, known from the Finish Patent document Fl 100286. Depending on the receive frequency band, a RF signal received by an antenna is coupled by means of switch 104 either to a first receive branch (DCS) or second receive branch (GSM).

If the received signal is in the DCS frequency band, it is conducted to band-pass filter 106, low-noise amplifier (LNA) 108 and bandpass filter 110. After that the signal is brought to block 112 which produces signal components having a 90-degree phase differences The in-phase component I and quadrature component Q are further conducted by means of switches 114 and 134 to mixers 116 and 136. The mixers get their mixing signals from a DOS synthesizer 140 the frequency of which corresponds to the received carrier frequency so that the mixing produces the in-phase and quadrature components of the complex baseband signal. The baseband signal is further processed in the receive (RX) signal processing unit, block 139.

If the signal received is a USM signal, switch 104 directs the received signal to the GSM branch which comprises, connected in series bandpass filter 126, low-noise amplifier 128, bandpass filter 130 and phase shifter 132 which generates two signals with a mutual phase difference of 90 degrees. The signals are further conducted by means of switches 114 and 134 to mixers 116 and 136 where the mixing frequency is now determined by a signal coming from the GSM synthesizer 150 via switch 161. The signals produced by the mixers are further conducted to the baseband RX signal processing unit 139.

The DCS synthesizer comprises In a known manner a phase-locked loop (PLL) which includes a voltage-controlled oscillator (VCO) 141 the output signal of which is amplified at amplifier 146 thus producing the synthesizer output signal. The frequency of the signal from oscillator 141 is divided by an integer Y in divider 142 and the resulting signal is conducted to phase comparator 143. Similarly, the frequency of the signal generated by reference oscillator 158 is divided by an integer X in divider 144 and conducted to phase comparator 143. The phase comparator produces a signal proportional to the phase difference of said two input signals, which signal is conducted to a low-pass filter (LPF) 145 producing a filtered signal that controls the voltage-controlled oscillator 141 The phase-locked loop described above operates in a known manner in which the output frequency of the synthesizer becomes locked to the frequency coming to the phase comparator from the reference frequency branch. The output frequency is controlled by varying the divisor Y.

The GSM synthesizer 150 comprises a voltage-controlled oscillator 150, amplifier 156, dividers 152 and 154, phase comparator 153 and a low-pass filter 155. The GSM synthesizer operates like the DCS synthesizer described above, but the output frequency of the GSM synthesizer corresponds to GSM frequency bands. In the transmitter part, a baseband complex transmit (TX) signal Is processed In a TX signal processing unit wherefrom the in-phase and quadrature components of the signal are conducted to mixers 162 and 182 that produce a carrier-frequency signal by multiplying the input signal by the mixing signal. If the transmission is at the DCS frequency, switch 161 selects the DCS synthesizer's output signal as the mixing signal. The carrier-frequency signal is conducted through switch 164 to the DCS branch where a 90-degree phase shift is first produced between the in-phase component and quadrature component, and the resulting signals are then summed, block 166. The resulting DCS signal is conducted to bandpass filter 168. amplifier 170, and bandpass filter 172. The RF signal thus produced is further conducted to the antenna 102 via switch 180.

If the transmission Is at the GSM frequency, the output signal of the GSM synthesizer is used as the mixing signal. The resulting carrier-frequency signal is conducted to the GSM branch in which it is processed in the same manner as in the DCS branch blocks 186, 188, 190 and 192. The RF signal thus produced is conducted to the antenna 102 via switch 180. One and the same antenna 102 can be used in both transmission and reception if the TX and RX circuits are coupled to the antenna through a duplex filter, for example. If the apparatus is designed to operate in two or more frequency bands, it needs separate filters for each frequency band.

The circuit arrangement described above has, however, some disadvantages. First, separate carrier-frequency signal branches in the receiver and m the transmitter add to the complexity, size and manufacturing costs of the transceiver. Second, each operating frequency band needs a separate synthesizer of its own.

An object of the invention is to provide a simple solution for realizing a programmable transceiver operating in a plurality of systems in such a manner that the aforementioned disadvantages related to the prior art can be avoided.

In the direct conversion based transceiver according to the invention signal processing can be performed using one and the same signal processing line regardless of the system. This is achieved using the signal processing steps set forth below.

The method according to the invention for processing signals received from different radio Interfaces of communication Systems Is characterized In that It comprises steps in which

-   -   a carrier-frequency signal Is received from a radio Interface,     -   the carrier-frequency signal is bandpass-filtered,     -   the filtered carrier-frequency signal is amplified,     -   an RX mixing signal at the receive frequency is generated,     -   a complex baseband signal is generated from the received         carrier-frequency signal by mixing it with the RX mixing signal,     -   the baseband signal generated is low-pass-filtered,     -   the baseband signal generated is amplified,     -   the baseband signal is converted digital, and     -   the baseband signal converted digital is processed to produce an         information signal encoded and modulated Into the received         signal.

The method according to the invention for processing signals transmitted to different radio Interfaces of communication systems Is characterized in that it comprises steps in which

-   -   a digital baseband quadrature signal is generated on the basis         of the information signal to be transmitted,     -   the digital baseband signal is converted analog,     -   a TX mixing signal at the transmit frequency is generated,     -   a carrier-frequency transmission signal is generated from the         baseband signal by mixing it with the TX mixing signal,     -   the carrier-frequency signal generated is amplified, and     -   the transmission signal is sent to the radio interface.

The direct-conversion receiver according to the invention operating at different interfaces of communication systems is characterized in that it comprises

-   -   antenna means for receiving a radio-frequency signal,     -   bandpass filter for filtering a carrier-frequency signal,     -   first RX amplifier for amplifying the filtered carrier-frequency         signal,     -   means for generating an RX mixing signal at the receive         frequency,     -   mixing means for generating a complex baseband signal from the         received signal

using me RX mixing signal,

-   -   low-pass filter for filtering the baseband signal,     -   second amplifier for amplifying the baseband signal,     -   analog-to-digital converter for converting the baseband signal         digital, and     -   means for processing the baseband signal converted digital to         produce an information signal encoded and modulated into the         received signal.

The direct-conversion transmitter according to the invention operating at different radio interfaces of communication systems is characterized in that it comprises

-   -   means for generating a digital baseband quadrature signal on the         basis of the information signal to be transmitted,     -   digital-to-analog converter For converting the baseband         transmission signal analog,     -   synthesizer for generating a TX mixing signal at the transmit         frequency,     -   mixing means for producing a signal at the carrier frequency         from the baseband

transmission signal using the TX mixing signal,

-   -   TX amplifier for amplifying the signal at the carrier frequency,         and     -   antenna means for transmitting the amplified transmission signal         at the carrier

frequency.

Other preferred embodiments of the invention are described in the dependent claims.

In the present invention, signal band limiting is advantageously performed at the baseband frequency so that there is no need for “steep” filters and, therefore, system-specific filter lines. Filtering can thus be performed as low-pass filtering using a filter with a controllable cut-off frequency. This way, it is possible to completely avoid separate system-specific channel filtering circuits.

To enable the generation of mixing frequencies of the different operating frequency bands by one and the same synthesizer it is advantageously used frequency division of the synthesizer output signal. If the synthesizer's operating frequency Is set higher than the frequencies used in the systems, it is possible to generate, in conjunction with the synthesizer frequency division, two mixing signals with a 90-degree phase difference, thus avoiding the need for phase shifters on the signal line and achieving a good phase accuracy.

Using the solution according to the Invention It Is possible to realize a general-purpose transceiver which is considerably simpler and more economical to manufacture than prior-art solutions. The circuit arrangement according to the invention requires only one TX signal branch and one RX signal branch Moreover, one and the same synthesizer may be used to generate the mixing signals. Furthermore, there is no need for channel filters operating at the radio frequency. Therefore, the circuitry can be easily integrated. Since the invention involves only a few components, the advantages of the transceiver according to the invention include small size and low power consumption.

The invention will now be described in more detail with reference to the accompanying drawing wherein

FIG. 1 shows a block diagram of a dual-band direct-conversion transceiver according to the prior art,

FIG. 2 shows In the form of block diagram a solution according to the invention for a direct-conversion transceiver operating in multiple systems.

FIG. 1 was already discussed in conjunction with the description of the prior art. Next, a transceiver according to the invention will be described, referring to FIG. 2.

FIG. 2 shows in the form of block diagram a transceiver according to the invention. A RF signal received through an antenna is conducted via matching circuits 1 to controllable bandpass filter 2. The matching circuits I may advantageously be controllable (AX) with respect to the operating frequency band. A controllable band-pass filter 2 may be advantageously realized using a plurality of bandpass filters so that the RF signal is conducted via switch elements controlled by a control signal FXI from the matching circuit 1 to the bandpass filter that corresponds to the selected operating frequency band. The bandpass filter may also be realized so as to be adjustable and tuneable by means or programming. The bandpass filtered carrier-frequency signal is further conducted to a low-noise amplifier 4, the gain of which is advantageously controllable. The control signal is marked GXI in the drawing. In addition to amplifier 4, it is also possible to have integrated amplifiers in connection with the bandpass filters.

The signal is then conducted to a mixer 5 in which the carrier-frequency signal is mixed with an RX mixing signal at the receive frequency to produce a baseband quadrature signal. The RX mixing signal is advantageously generated by a synthesizer 10 the output signal frequency of which is divided by a divider 11 so as to correspond to the selected receive frequency. The synthesizer 10 operates in a similar manner as the synthesizers depicted in FIG. 1. Thus it comprises a voltage-controlled oscillator VCO which produces an output signal. The frequency of the VCO output signal is divided by S1 in a divider in the phase locked loop PLL. The resulting signal is conducted to a first input of a phase comparator In the phase-locked loop. Similarly, the frequency of a signal generated by a reference oscillator in the phase-locked loop PLL is divided by an integer and conducted to a second input of the phase comparator. The phase comparator produces a signal which is proportional to the phase difference of the two input signals and conducted to a low-pass filter, and the filtered signal then controls the voltage-controlled oscillator VCO. The output frequency is controlled by varying the divisor SI.

The synthesizer output signal is divided in divider 11 by NI so that the RX mixing signal corresponds to the selected receive frequency band. The output frequency of the synthesizer may be e.g. in the 4 GHz band, so that with 2-GHz systems the synthesizer output frequency is divided by two, and with 1-GHz systems it is divided by four (N1). This way, systems operating in the 1-GHz and 2-GHz bands can be covered with a synthesizer the operating frequency band of which is narrow with respect to the operating frequency.

To produce a quadrature baseband signal the mixer needs two mixing signals with a phase shift of 90 degrees Phase-shifted components may be produced by a phase shifter in connection with the mixer or they may be produced as quotients generated already in the frequency divider 11, thus achieving an accurate phase difference. Therefore, it is advantageous to use a synthesizer operating frequency which is a multiple of the highest system frequency.

The in-phase component 1 and quadrature component Q from the mixer 5 are further conducted to low-pass filters 6. The higher cut-off frequency of the low-pass filters is advantageously controllable with control signal FX3. Thus the filtering can be performed at a bandwidth corresponding to the selected radio interface, and since the filtering is performed at baseband, it is easy to get the filtering function steep. Also, no strict demands are set on the bandpass filtering (2) of the RF signal.

The baseband signal is further conducted to a gain control block 7 which possibly includes an offset voltage correction block. On the other hand, considering the broad bandwidth of the COM_A system, the offset voltage can easily be removed by high-pass filtering. The amplifier advantageously realizes automatic gain control (AGC). Finally, the signal is convened digital in an analog-to-digital converter 8, and the digital baseband signal is further processed in a digital signal processor (DSP) 9. Channel filtering may also be performed digitally in the DSP, whereby the low-pass filtering of the baseband signal may be performed using a fixed cut-off frequency. Then, however, the dynamics of the analog-to-digital converter must be considerably better.

In the transmitter part, a quadrature baseband signal is first digitally generated in block 9 on the basis of the information signal to be sent. The components of the digital signal are converted analog by digital-to-analog converters 14, whereafter the analog signals are low-pass filtered by low-pass filters 15. Advantageously, the cutoff frequency of the low-pass filters can be controlled with control signal FX4 so as to correspond to the specifications of the selected radio interface.

A TX mixing signal at the carrier frequency is generated by a synthesizer 13 and divider 12. The synthesizer 13 operates in a similar manner as the synthesizer 10 in the receiver pan. Moreover, the synthesizers may share a reference oscillator. The frequency of the synthesizer output signal is controlled with control signal 52 within the synthesizer's operating frequency range. The frequency of the output signal from synthesizer 13 is divided in divider 12 so as to correspond to the selected transmission frequency band. Components phase-shifted by 90 degrees are generated from the TX mixing signal in order to perform complex mixing in mixer 16. The phase-shifted components may be generated in the same way as in the receiver part.

The signal at the carrier frequency is then amplified in an amplifier 17, the gain of which is advantageously controllable in order to set the transmission power and realize automatic gain control (AGC). The control signal is marked GX3 in FIG. 2. The signal is then conducted to a power amplifier 18. The operating frequency band of the power amplifier is advantageously selectable with control signal BX. This can be achieved e.g. such that the amplifier comprises partly separate signal lines for the different operating frequency bands.

The RF signal generated is filtered by a bandpass filter 3. The pass band of the bandpass filter is advantageously controllable with control signal FX2. This can be realized in the same way as in the receiver part. The receiver and transmitter part filters 2 and 3 are advantageously realized in duplex filter pairs for each transmit-receive frequency band associated with a given system. The filters may advantageously be surface acoustic wave (SAW) or bulk acoustic wave (BAW) filters so that several filters with their switches may be attached to one component.

The control signals in the mobile station transceiver according to FIG. 2 are preferably generated in a control block of the mobile station which advantageously comprises a processing unit such as a microprocessor. The control block generates the signal on the basis of a system switch instruction input from the keypad of the mobile station, for example. System selection may be e.g. menu-based so that the desired system is selected by choosing it from a displayed menu by pressing a certain key on the keypad. The control block then generates the control signals that correspond to the selected system. The system switch instruction may also come via the mobile communication system in such a manner that data received from the system may include a system switch instruction on the basis of which the control block performs the system switch. Advantageously, a control program is stored in a memory unit used by the control block, which control program monitors the received data and, as it detects a system switch instruction in the data, gives the control block an instruction to set the control signals into states according to the selection instruction.

The implementation of the blocks described above is not illustrated in more detail as the blocks can be realized on the basis of the information disclosed above, applying the usual know-how of a person skilled in the art.

Above it was described embodiments of the solution according to the invention. Naturally, the principle according to the invention may be modified within the scope of the invention as defined by the claims appended hereto, e.g. as regards implementation details and fields of application. It is especially noteworthy that the solution according to the invention may be well applied to communication systems other than the mobile communication systems mentioned above. Apart from the cellular radio interface proper, the solution may be used to realize e.g. a GPS receiver for the location of a mobile station or other apparatus. Furthermore, the operating frequencies mentioned are given by way of example only, and the implementation of the invention is in no way restricted to them.

It is also noteworthy that the solution according to the invention may be applied to all current coding techniques such as the narrow-band FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access), as well as me broadband CDMA (Code Division Multiple Access) technique. In addition, the solution according to the invention may be used to realize an FM (Frequency Modulation) receiver.

Below is a table listing some of the so-called second generation mobile communication systems to which the present invention may be applied. The table shows the most important radio interface related characteristics of the systems.

GSM PDC DECT Digital PHS Personal Global System Personal European Handy CELLULAR IS-95 US for Mobile Digital Cordless Phone SYSTEM AMPS IS-54/-136 CDMA Communications DCS 1800 Cellular Telephone System RX FREQ. 869-894 869-894 869-894 935-960 1805-1880  810-826, 1880-1900 1895-1918 (MHz) 1429-1453 TX FREQ. 824-849 824-849 824-849 890-915 1710-1785 940-956 1880-1900 1895-1918 (MHz) 1477-1501 RF BAND- 25 MHz 25 MHz 25 MHz 25 MHz 75 MHz 16 MHz 20 MHz 23 MHz WITH 24 MHz MULTIPLE FDMA TDMA/ CDMA/ TDMA/ TDMA/ TDMA/ TDMA/ TDMA/ ACCESS FDMA FDMA FDMA FDMA FDMA FDMA FDMA METHOD DUPLEX FDD FDD FDD FDD FDD FDD TDD TDD METHOD NUMBER OF 832 832, 20, 124, 374, 1600, 10, 300, CHANNELS 3 users/ 798 users/ 8 users/ 8 users/ 3 users/ 12 users/ 4 users/ channel channel channel channel channel channel channel CHANNEL 30 KHz 30 kHz 1250 kHz 200 kHz 200 kHz 25 kHz 1,728 MHz 300 kHz SPACING MODULATION FM π/4DQPSK QPSK/ GMSK 0.3 GMSK 0.3 π/4 DQPSK GFSK 0.3 π/4 DQPSK OQPSK Gaussian Gaussian Gaussian filter filter filter CHANNEL — 48.6 kb/s 1.2288 Mb/s 270.833 kb/s 270.833 kb/s 42 kb/s 1.152 MB/S 384 kb/s BIT RATE

Below is another table listing some of the so-called third generation mobile communication systems to which the present invention may be applied. The table shows the most important radio interface related characteristics of the system.

CELLULAR SYSTEM WCDMA RX FREQ. (MHz) 2110-2170 1900-1920 TX FREQ. (MHz) 1920-1980 1900-1920 MULTIPLE ACCESS CDMA TDMA METHOD DUPLEX METHOD FDD TDD CHANNEL SPACING 5 MHz 5 MHz MODULATION QPSK CHANNEL BIT RATE 144 kb/s in rural outdoor, 500 kb/s in urban outdoor and up to 2 Mb/s in indoor 

1. A direct-conversion transceiver capable of operating with different radio interfaces including a first radio interface conforming to a code division multiple access (CDMA) system and a second radio interface conforming to a time division multiple access (TDMA) system, comprising: a first controllable bandpass filter configured to filter a received signal according to a control signal that selects one of a plurality of passbands corresponding to one of the different radio interfaces, wherein the first controllable bandpass filter has a signal path common to both the first radio interface and the second radio interface; a low-noise amplifier configured to amplify the filtered received signal according to a control signal that controls an amount of gain, wherein the low-noise amplifier has a signal path common to both the first radio interface and the second radio interface; a first programmable synthesizer configured to generate a first mixing signal according to a control signal corresponding to the selected one radio interface, wherein the first programmable synthesizer has a signal path common to both the first radio interface and the second radio interface; a first frequency divider coupled to the first programmable synthesizer and configured to divide a frequency of the first mixing signal by two to provide a first divided frequency signal according to a control signal corresponding to the selected one radio interface; a first mixer coupled to the low-noise amplifier and configured to mix the amplified and filtered received signal with the first divided mixing signal to produce a first baseband quadrature signal, wherein the first mixer has a signal path common to both the first radio interface and the second radio interface and wherein the first mixer produces the first basedband quadrature signal on the basis of two 90-degree phase-shifted components produced from the first frequency divider and is operable to process either a TDMA signal or a CDMA signal; a first low-pass filter coupled to the first mixer and configured to low-pass filter the first baseband quadrature signal according to a control signal corresponding to the selected one radio interface, wherein the first low-pass filter has a signal path common to both the first radio interface and the second radio interface and is operable to process either a TDMA signal or a CDMA signal; a first gain-controlled amplifier coupled to the first low-pass filter and configured to provide gain-controlled amplification of the first low-pass filtered baseband quadrature signal, wherein the first gain-controlled amplifier has a signal path common to both the first radio interface and the second radio interface and is operable to process either a TDMA signal or a CDMA signal; an analog-to-digital converter coupled to the first gain-controlled amplifier and configured to convert to digital form an output of the first gain-controlled amplifier; a digital signal processor configured to receive digital output from the analog-to-digital converter and to further process said digital output; a digital-to-analog converter coupled to the digital signal processor and configured to receive a second baseband quadrature signal therefrom and to provide analog output signals; a second low-pass filter coupled to the digital-to-analog converter and configured to low-pass filter the analog output signals from the digital-to-analog converter according to a control signal corresponding to the selected one radio interface, wherein the second low-pass filter has a signal path common to both the first radio interface and the second radio interface and is operable to process either a TDMA signal or a CDMA signal; a second programmable synthesizer configured to generate a second mixing signal according to a control signal corresponding to the selected one radio interface, wherein the second programmable synthesizer has a signal path common to both the first radio interface and the second radio interface; a second frequency divider coupled to the second programmable synthesizer and configured to divide a frequency of the second mixing signal by two to provide a second divided frequency signal according to a control signal corresponding to the selected one radio interface; a second mixer coupled to the second low-pass filter and configured to mix signals from the second low-pass filter and the second frequency divider to produce a carrier-frequency transmission signal, wherein the second mixer has a signal path common to both the first radio interface and the second radio interface and wherein the second mixer produces the carrier-frequency transmission signal on the basis of two 90-degree phase-shifted components produced from the second frequency divider and is operable to process either a TDMA signal or a CDMA signal; a second gain-controlled amplifier coupled to the second mixer and configured to control gain according to a control signal corresponding to the selected one radio interface, wherein the second gain-controlled amplifier has a signal path common to both the first radio interface and the second radio interface and is operable to process either a TDMA signal or a CDMA signal; a power amplifier coupled to the second gain-controlled amplifier and configured to produce an amplified output using a frequency band determined on the basis of a control signal corresponding to the selected one radio interface, wherein the power amplifier has a signal path common to both the first radio interface and the second radio interface; a second controllable bandpass filter configured to filter an output of the power amplifier according to a control signal that selects one of a plurality of passbands corresponding to the selected one radio interface, wherein the second controllable bandpass filter has a signal path common to both the first radio interface and the second radio interface; and a microprocessor configured to generate one or more control signals to cause selection of the selected radio interface.
 2. Apparatus comprising: a processor configured to generate a digital baseband quadrature signal on the basis of an information signal to be transmitted; a digital-to-analog converter configured to convert the digital baseband quadrature signal to an analog baseband transmission signal; a synthesizer configured to generate a transmission mixing signal; a mixer configured to produce a carrier frequency signal based on the analog baseband quadrature signal and the transmission mixing signal; a controllable gain transmitter amplifier configured to amplify the carrier frequency signal; a controllable low-pass filter configured to filter the analog baseband transmission signal and to perform channel filtering according to a selected one of a plurality of radio interfaces; a frequency divider configured to divide a frequency of an output signal generated by the synthesizer and to provide components with a 90-degree phase difference between them, wherein said frequency division is a division at least by two; and wherein the controllable gain transmitter amplifier and the controllable low-pass filter are configured to process radio frequency signals associated with different radio interfaces of communication systems including a time division multiple access scheme and a code divisional multiple access scheme; and wherein the controllable gain transmitter amplifier is configured to process transmitter signals that have earlier passed through the controllable low-pass filter.
 3. The apparatus of claim 2, wherein the controllable low-pass filter performs channel filtering in a digital manner.
 4. The apparatus of claim 2, wherein the controllable gain transmitter amplifier performs automatic gain control.
 5. The apparatus of claim 2, further comprising means for controlling an operating frequency band of the controllable gain transmitter amplifier.
 6. The apparatus of claim 2, further comprising: a bandpass filter configured to filter the amplified transmission signal at the carrier frequency; and means for selecting a pass band of the bandpass filter such that it corresponds to the transmission frequency.
 7. The apparatus of claim 2, wherein the radio interfaces have different operating frequencies.
 8. The apparatus of claim 2, wherein the apparatus is configured to use the same mixer, in a direct conversion transmitter portion of the apparatus, to process radio frequency signals associated with different radio interfaces.
 9. The apparatus of claim 2, wherein the system employing a time divisional multiple access method is Global System for Mobile Communication (GSM) and the system employing a code division multiple access method is wideband CDMA.
 10. The apparatus of claim 2, wherein a transmitter portion of the apparatus is configured to use the same synthesizer for processing radio frequency signals associated with different radio interfaces.
 11. The apparatus of claim 2, wherein the output frequency of the synthesizer is controlled with a control signal.
 12. The apparatus of claim 11, wherein the synthesizer comprises a voltage controlled oscillator (VCO) and a phase-locked loop (PLL).
 13. The apparatus of claim 12, wherein the synthesizer is arranged to produce an output signal, the frequency of which is divided by the control signal in a divider in the phase-locked loop (PLL), wherein the synthesizer's output frequency is controlled by varying the divisor.
 14. The apparatus of claim 2, further comprising a control program configured to generate system-switch control signals that correspond to the selected one radio interface.
 15. The apparatus of claim 14, wherein the system-switch control signals are set on the basis of a manual system switch instruction input or system switch instruction within received data that is monitored.
 16. The apparatus of claim 2, wherein the carrier frequency signal is amplified by a power amplifier having an operating frequency band selectable corresponding to the selected one radio interface.
 17. The apparatus of claim 2, further comprising a receiver section comprising: a bandpass filter configured to filter the carrier-frequency signal, a first receiver amplifier configured to amplify the filtered carrier-frequency signal, means for generating a receiver mixing signal at the receive frequency, means for generating a complex baseband signal from the received signal by means of the receiver mixing signal, a low-pass filter configured to filter the complex baseband signal, a second amplifier configured to amplify the complex baseband signal, an analog-to-digital converter configured to convert the baseband signal to digital form, and means for processing the digital baseband signal so as to produce an information signal.
 18. The apparatus of claim 17, wherein the second amplifier is configured to perform automatic gain control.
 19. The apparatus of claim 17, wherein the second amplifier comprises an offset voltage correction circuit.
 20. A method comprising: generating a digital baseband quadrature signal on the basis of an information signal; converting the digital baseband signal to an analog form of the digital baseband signal; low-pass filtering the analog form of the digital baseband signal using a controllable cut-off frequency to produce a low-pass filtered signal, wherein the cut-off frequency is controlled so as to correspond to specifications of a selected one of a plurality of a different radio interfaces; generating a mixing signal at a transmission frequency; generating a carrier-frequency transmission signal by mixing the low-pass filtered signal with the mixing signal; amplifying the carrier-frequency transmission signal using a transmitter amplifier to produce a transmission signal, wherein an operating frequency band of the transmitter amplifier is selectable by a control signal; and transmitting the transmission signal to the selected one of the plurality of different radio interfaces.
 21. The method of claim 20, wherein a first of the plurality of different radio interfaces comprises a code division multiple access radio system and a second of the plurality of different radio interfaces comprises a time division multiple access radio system.
 22. The method of claim 21, wherein the selected one of the plurality of radio interfaces comprises a time division multiple access radio system.
 23. The method of claim 20, wherein each of the plurality of different radio interfaces operates at a different transmission frequency.
 24. Apparatus comprising: means for generating a digital baseband quadrature signal on the basis of an information signal; means for converting the digital baseband signal to an analog form of the digital baseband signal; means for low-pass filtering the analog form of the digital baseband signal using a controllable cut-off frequency to produce a low-pass filtered signal, wherein the cut-off frequency is controlled so as to correspond to specifications of a selected one of a plurality of a different radio interfaces; means for generating a mixing signal at a transmission frequency; means for generating a carrier-frequency transmission signal by mixing the low-pass filtered signal with the mixing signal; means for amplifying the carrier-frequency transmission signal to produce a transmission signal, wherein an operating frequency band of the means for amplifying is selectable by a control signal; and means for transmitting the transmission signal to the selected one of the plurality of different radio interfaces.
 25. The apparatus of claim 24, further comprising means for implementing channel filtering in a digital manner.
 26. The apparatus of claim 24, wherein the means for generating a mixing signal comprises a synthesizer and a controllable frequency divider configured to divide a frequency of an output signal generated by the transmission synthesizer.
 27. The apparatus of claim 24, further comprising means for controlling a gain of the means for amplifying the carrier-frequency transmission signal.
 28. The apparatus of claim 24, wherein the means for amplifying performs automatic gain control.
 29. The apparatus of claim 24, further comprising: a bandpass filter configured to filter an amplified transmission signal at the carrier frequency; and means for selecting a passband of the transmitter bandpass filter so that it corresponds to the transmission frequency. 